Various conventional apparatuses using the action of a radio interferometer with a compensation channel for extracting a modulated component of a radio frequency (RF) signal, corresponding to the frequency of human's heart rate or breath, are known in the prior art (PCT/DE 95/00062; U.S. Pat. No. 4,967,751; DE A 4241664; IEEE Transactions on Instrumentation and Measurement, Bd.40, No.4, August 1991, New York, US, p.p.747-760, Chuang, Chen, Chen, "Automatic Clutter-Canceler for Microwave Life-Detection Systems"). The apparatuses are employed in noncontact diagnostics and also useful in detecting a living object under obstructions formed, for instance, after earthquakes, accidents or avalanches. The main constraint on the use of the conventional apparatuses for detecting alive people under obstructions is that it is impossible to select a search zone by distance, and the apparatuses are highly susceptible to the presence of an operator handling them.
A conventional electronic system for detecting a living object comprises a modulator and a transmitter including an oscillator, a power splitter and a transmitting antenna, a receiver including a receiving antenna, a microwave receiver, a preamplifier/modulator and signal processing unit, a second signal output of the power splitter being connected to a control input of the microwave receiver, and one of control outputs of the unit being connected to the control input of the microwave receiver (DE, A, 42416164).
In the above apparatus, a dc component in a reflected signal is compensated for at an input of the microwave receiver prior to amplification, this resulting in additional noise introduced by compensation means, and the modulator serves only to provide a possibility of amplification of a signal component modulated by a living object, lying outside the noise region 1/f. The apparatus, therefore, has an additional source of amplitude and phase noise at its input. This restricts a minimal level of the received signal and impairs sensitivity, prohibiting the determination of a distance to a living object with such an apparatus.
Disclosed in IEEE Transactions on BME, V.33, Jul. 7, 1986, Kun-Mu Chen, D.Misra, H.-R. Chuang "An X-band Microwave Life-Detection System" is a method for locating a living object, including the steps of radiating a radio frequency (RF) signal, receiving a return RF signal at the place of the RF signal radiation, extracting a component of the received RF signal, which is modulated in amplitude and phase corresponding to the frequency of a heart rate and/or breath of a living motionless object, and judging, on the basis of the extracted component, of detecting a living object.
The above method provides for a compensation of a microwave signal reflected from a mass of motionless objects, extraction and analysis of a variable component of the reflected signal associated with the aforementioned human life signs by comparison of the reflected signal amplitude and phase with their constant values in the radiated signal. The reason for the use of compensation is the particularity of the apparatus for implementing the method. With a microwave signal reflected from a light-tight obstruction and other highly-reflecting motionless objects available within the region of a pattern of a transmitting/receiving antenna of the apparatus, and as the result of direct penetration of a signal portion from the transmitter circuit to the receiver circuit, a high-power background signal comes to the latter, this signal having a constant amplitude and phase and being capable of leaving the linearity range of the receiver's gain. On the other hand, the RF signal reflected from a living object is twice attenuated by the obstruction at the forward passage of the radiated RF signal and the return passage of the reflected RF signal. The signals are also attenuated by a distance to a living object located behind the obstruction, this attenuation being proportional to the forth power of the distance to the object. Moreover, the power level of the desired signal modulated component carrying information of the object vital activity is smaller than that of a total signal reflected from the object by a factor of 10 to 1,000.
These circumstances require that the receiving circuit should transmit a very large dynamic range of energies of the reflected RF signal, as great as 100-150 dB. The dynamic range of the desired signal energy is generally no greater than 90 dB. Hence, it is necessary to delete all useless information of motionless objects upstream of the first amplifier in the receiver by removing the background signal. This is attained by addition of a compensating microwave circuit, at an output of which a signal approximately equal in amplitude and opposite in phase to the background signal is provided by controlling parameters of the microwave circuit. The compensating signal is combined with the signal received at the receiver input in a passive combiner. The combiner outputs a power which is approximately equal to a difference of the received and background signal powers. The performance quality of the apparatus is mainly defined by the quality of this compensation, its depth, and fluctuations in the compensation circuits and the circuits controlling its parameters.
The prior art method enables effective extraction of information associated with human's vital activity, however, its abilities in performing search and rescue works are limited. The method does not restrict the search zone by distance for locating a human, and it is very susceptible to the presence of an operator handling the apparatus. The above deficiencies are caused by radiation of a continuous microwave signal which is not modulated in time and which, being reflected, combines all information of the objects disposed both within the region of the antenna pattern from its aperture plane to a maximum detection distance, and within the region of the antenna side and back lobes when the operator is nearby.
The impossibility to restrict a search zone, i.e. to preset a minimum and maximum detection distance, results in interference from people and equipment operating at the accidence site in the immediate vicinity of the search zone, and from the operator, since the high sensitivity required for extracting a modulated component is sufficient to respond to all living objects located in air space at a considerable distance or to the operator due to availability of side and back lobes of the antenna pattern. Further, when clearing away the obstruction and recovering a living object, technical means employed may injure the living object, since the conventional method does not allow the determination of a distance to a living object.
A conventional microwave locator comprises a modulator and a transmitter including an oscillator, a power splitter and a transmitting antenna having signal inputs connected in series to signal outputs, the transmitter being adapted to radiate a RF signal with modulation, a receiver including a receiving antenna, a microwave receiver, a preamplifier/demodulator and a signal processing unit, all having signal outputs connected in series with inputs, the receiver being adapted to receive a reflected RF signal modulated by a component of human's heart rate and/or breath and extract the component at the output of the preamplifier/demodulator, a second signal output of the power splitter being connected to a control input of the microwave receiver, a first control output of the modulator being connected to the transmitter, and a second and third outputs of the modulator being connected to a first and second control inputs of the preamplifier/demodulator (U.S. Pat. No. 4,958,638).
An advantage of this apparatus over the previously mentioned prior art is in its high sensitivity in determining parameters of the modulated component, which is provided by the absence of a high self-noise level in the microwave receiver, caused by imperfect compensation at its input, introducing additional fluctuations in amplitude and phase of the compensated amplified signal. This apparatus, however, can be employed only for noncontact diagnostics. The apparatus lacks the ability to arbitrarily select a search zone and determine a distance to a living object. Moreover, in sounding distant spatial areas by the RF signal, the apparatus has dead sensitivity zones associated with single-channel phase detection employed therein.